The Pickpocket ability in general for me. While it seems a good idea, in the competitive environment it is only good if you use a comsumable item, which most users of it require a life orb for extra damage, or a choice item of some kind.
I won't even mention how useful it is in game Mr.LetsGiveItemsBackToTheOpponentAfterABattle. The only other use it has after that is stealing items from Pokemon in the wild, which Frisk-Thief users do much better.
3DS Friend Code - 4356-0021-1824
Safari Type - Bug
Pokemon - Beautifly, Combee, Heracross